An essential item in any proper navigation kit, the Aluminum Arm Parallel Ruler is used to plot direction on your nautical chart. The aluminum parallel ruler has two hinged parallel edges that maintain the same angle. By alternating the moving edge and securely holding down the non-moving edge, you can move the aluminum parallel ruler about the chart while still maintaining the same angle. Aluminum arms and handles. 18" long. 1/4" thick. Lifetime warranty.